In co-pending application Ser. No. 616,673 entitled Method of Use and Electrical Equipment Utilizing Insulating Oil Consisting of a Saturated Hydrocarbon Oil, filed on Sept. 25, 1975, now U.S. Pat. No. 4,082,866 certain highly refined petroleum oils or mineral oils were disclosed which were considered sufficiently non-flammable to serve as insulating oil substitutes for polychlorinated biphenals in electrical equipment which is operated at moderate ambient temperatures of 0.degree. C. to about 40.degree. C. These oils comprised straight and branched chain aliphatic paraffinic hydrocarbons which have a molecular weight of about 500 to about 700, preferably about 600, and a fire point above 200.degree. C. In this regard, a suitable aliphatic paraffinic oil disclosed was a dual treat base oil which is a solvent treated deeply hydrogenated bright stock and is predominantly a paraffinic oil with a molecular weight in excess of 600.
It has been found that where the insulating fluid is of a hydrocarbon origin and where paper or other cellulosic material is used to provide physical separation of the conductive elements, decomposition of the paper or cellulosic material produces water. Additional water is introduced by the exchange of air over the oil in the normal thermal cycling of the equipment, if vented. Solubility of water in the dielectric insulating fluid is therefore of utmost importance.
Thermal aging of the insulation system, whether by normal or abnormal means, produces a quantity of water which must be absorbed and maintained in the insulating fluid. If the quantity of water is sufficient to saturate the insulating fluid, the excess water produced will form a second phase with a high dielectric constant and a low dielectric strength. This is a demonstrated sequence in the path leading to the early failure of these systems.
It has been found that in using a aliphatic paraffinic hydrocarbon as the insulating fluid as described above, the solubility of water is much less than that experienced with conventional transformer oil.
In testing some of the Askarel substitutes to increase their fire point for this use, a chemical known as tri-cresyl phosphate, manufactured by the Stauffer Chemical Co. and sold under the trade name "SYN-O-AD8484", was tested to increase fire point. This additive was found to have limited solubility in the above described paraffinic hydrocarbon oil (approximately 2.5% by volume at normal room temperature). At that concentration, this material offered no advantage with respect to improving the flammability properties of the blend. However, it was restrained in its negative effects upon the electrical properties of the fluid blend. This material is described by Stauffer Chemical Co. as hydrolizing slowly under wet, alkaline conditions at ambient temperatures and has shown a surprising stability under accelerated aging conditions.
When the question of water solubility of the aliphatic paraffinic hydrocarbon oils was raised, this was one of the materials which was looked at as providing a solution to that problem. This, in spite of tri-cresyl phosphate being a phosphate ester and thus labeled as being unstable in the presence of water. It is virtually universally accepted by the experts in hydrocarbon dielectric fluids that phosphate esters as a class hydrolize in the presence of water to form phosphoric acid. Very low concentrations of phosphoric acid in a dielectric insulating fluid would result in a decrease in its electrical resistivity by several orders of magnitude. This would inevitably lead to early failure of the equipment.